The present invention relates to a fuel cell comprising a solid polymer electrolyte used for portable power sources, electric vehicle power sources, domestic cogeneration systems, etc.
A fuel cell comprising a solid polymer electrolyte generates electric power and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air. This fuel cell is basically composed of a polymer electrolyte membrane for selectively transporting hydrogen ions, and a pair of electrodes formed on both surfaces of the polymer electrolyte membrane. The electrode usually comprises a catalyst layer which is composed mainly of carbon particles carrying a platinum group metal catalyst and a diffusion layer which has both gas permeability and electronic conductivity and is formed on the outer surface of the catalyst layer.
Moreover, gaskets or gas sealing materials are arranged on the outer periphery of the electrodes with the polymer electrolyte membrane therebetween so as to prevent a fuel gas and an oxidant gas from leaking out or prevent these two kinds of gases from mixing together. The gaskets are combined integrally with the electrodes and polymer electrolyte membrane beforehand. This is called “MEA” (membrane electrode assembly). Disposed outside the MEA are conductive separators for mechanically securing the MEA and for connecting adjacent MEAs electrically in series. The separators have a gas flow channel for supplying a reaction gas to the electrode surface and for removing a generated gas and an excess gas at a portion to come in contact with the MEA. Although the gas flow channel may be provided separately from the separators, grooves are usually formed on the surfaces of the separators to serve as the gas flow channel.
In order to supply the gas to such grooves, it is necessary to use a piping jig, called “manifold”, which branches out, depending on the number of the separators, into the grooves of the respective separators from a gas supply pipe. This type of manifold, directly connecting the gas supply pipe to the grooves of the separators, is specifically called “external manifold”. There is also another type of manifold, called “internal manifold”, which has a more simple structure. In the internal manifold, the separators with the gas flow channel formed thereon are provided with through holes, called “manifold aperture”, which are connected to the inlet and outlet of the gas flow channel, and the gas is supplied directly from the manifold apertures.
Since the fuel cell generates heat during operation, it needs cooling with cooling water or the like to keep good temperature conditions. Thus, a cooling section for flowing the cooling water therein is generally inserted between the separators for every one to three cells, and the cooling section is often formed by providing the backside of the separator with a cooling water flow channel. In a general structure of the fuel cell, the MEAs, separators and cooling sections, as described above, are alternately stacked to form a stack of 10 to 200 cells, and the resultant cell stack is sandwiched by end plates with a current collector plate and an insulating plate interposed between the cell stack and each end plate and is clamped with clamping bolts from both sides.
In such a polymer electrolyte fuel cell, the separators need to have a high conductivity, high gas tightness, and high corrosion resistance to oxidation/reduction reactions of hydrogen/oxygen. For such reasons, conventional separators are usually formed from carbon materials such as graphite and expanded graphite, and the gas flow channel is formed by cutting the surface of the separator or by molding in the case of expanded graphite separator.
The fuel cell produced in the above-described manner is supplied with the fuel gas, oxidant gas and cooling water to examine the performance of the fuel cell or of a unit cell of the fuel cell.
The prior art fuel cell, comprising the cell stack in which the MEA is disposed between two conventional conductive separators, poses a large problem resulting from the separators. Specifically, in such a fuel cell, the gasket arranged on the periphery of the MEA is pressed to fall into the gas flow channel of one of the two separators due to the clamping pressure of the fuel cell, thereby to form a clearance between the gasket of the MEA and the other separator. Such a clearance is liable to occur at the ends of the gas flow channel in the vicinity of the manifold apertures. Through the clearance, two kinds of gases mix with each other, resulting in deterioration of cell performance. Also, the mixing of the gasses may cause explosion or firing, thus inviting dangerous situations.